Steve said:
What are the advantages of using a relay instead of an SCR or triac?
Steve Wimark
Mountain View High School, Mountain View, CA
Others have already answered regarding relays being
cheaper at high currents.
In addition, an SCR or triac will put more resistance in the
circuit than a relay will. Sometimes that higher resistance
will prevent proper circuit operation, so a relay rather than
and SCR or triac is needed. Also, that resistance will develop
heat when current flows through it, and it will reduce the voltage.
The amount of resistance is very small, but when high currents
are involved, it becomes a significant factor. A relay will
also put some resistance in the circuit - but FAR less than a
triac or SCR, so the heat produced is way less, and the voltage
reduction is insignificant.
This gets a little more complicated:
Relays function differently than SCR's. When you send
a signal to turn an SCR on, it stays on until one of
two things happens - either the circuit that the SCR
is controlling stops drawing sufficient current to
keep the SCR conducting, or a different signal is sent
to the SCR to turn it off. On the other hand, a relay
will stay on only as long as the signal telling it to turn
on is present. (And as long as that signal is present, the
relay will stay on, even if the circuit it is controlling
is drawing no current at all.) If you want it to stay on,
you either have to ensure that that relay's turn-on signal
will stay on, or use a special kind of relay called a latching
relay.
A triac is similar to an SCR. However, a triac used in an
AC circuit will reach one of the turn off conditions
I mentioned for the SCR "automatically" since the
circuit is AC. The circuit the triac controls will stop
drawing current when the AC sine wave reaches 0 volts.
Any of these differences could make one device more
preferable than the other for the particular circuit.
Now, a little more complexity:
An inductive device such as a transformer, a motor, a fluorescent
light etc. produces an "inductive kick" when it is turned
off. Solid state devices can be burned out by these,
and triacs can fail to turn off even when not damaged by them.
So with SCR's and triacs controlling inductive devices,
special design consideration has to be given to ensure
both that the circuit will work and that it won't be damaged.
A relay can also be damaged, but it is far less susceptible
to damage form the inductive kick than an SCR or triac, all other
conditions being equal. Where this shows up in the consumer
world is in dimmers that are to be used for incandescent
loads only. For example, if you want to dim low voltage halogen
lights which get their power from a transformer, you can't use
a regular dimmer. Some remote control systems such as X-10
or Smarthome have "appliance modules" and "light modules".
The "light modules" have a triac that turns the lights on
or off (and sometimes dims them) while the "appliance modules"
use a relay. When I added low voltage outdoor landscape
lights to the circuit for my outdoor lighting, the remote
(X-10) system could no longer turn the lights off! The low
voltage system uses a transformer, and at turn-off the
inductive kick it produces prevented the triac from turning
off.
